The invention relates to a power supply circuit for a telephone apparatus, provided with an input terminal, an output terminal and a reference terminal whose potential is intended to serve as a reference potential for the whole circuit, the input terminal and the reference terminal being intended to be connected via a rectifier bridge to a telephone line and to receive and supply a current referred to as power supply current, said power supply circuit comprising:
a capacitance arranged between the output terminal and the reference terminal, PA1 a first transistor with its collector-emitter path arranged between the first input terminal and the output terminal, PA1 a second transistor with its collector-emitter path arranged between the input terminal and the reference terminal, and PA1 a control module intended to make a comparison between the potential of the input terminal and that of the output terminal and to introduce a conduction current and a current complementary thereto into the bases of the first and the second transistor, respectively, the values of said currents depending on the result of the comparison and their sum being referred to as the bias current. PA1 a current mirror, of which an input branch is connected to a first current source supplying a first current having a predetermined value, and an output branch is connected to the interconnected emitters of the transistors of the differential pair, PA1 a current divider having an input intended to receive a signal which is representative of the power supply current, and an output which is intended to supply a current having a value which is a fraction of that of the power supply current, PA1 a second current source intended to supply a second current of a predetermined value, connected between the output of the current divider and the reference terminal, and PA1 a fifth transistor having its base connected to the output of the current divider, and its collector-emitter path arranged between the input branch of the current mirror and the reference terminal. PA1 a rectifier bridge having two AC terminals intended to be connected to a telephone line, a first and a second DC terminal intended to supply and receive, respectively, PA1 a rectified current referred to as line current, an interface module connected between the DC terminals and particularly intended to process the information conveyed by the line current, and PA1 a microcontroller intended to receive and manage commands given by a user of the apparatus, provided with a positive power supply terminal and a negative power supply terminal which is connected to one of the DC terminals of the rectifier bridge, PA1 telephone apparatus characterized in that it also comprises PA1 a power supply circuit as described above, whose input terminals are connected to the DC terminals of the rectifier bridge and whose output terminal is connected to the positive power supply terminal of the microcontroller, and PA1 a voltage regulation module connected between the output terminal of the power supply circuit and the reference terminal, intended to maintain the average value of the potential of said output terminal constant.
A power supply circuit of this type is known from European patent application 0 681 389 A2. The function of this circuit is to supply a voltage from its output terminal, intended as a power supply voltage for other circuits within the telephone apparatus after having been regulated by means of an appropriate device. In this power supply circuit, the first transistor is turned on by means of the conduction current when the potential of the output terminal is lower than that of the input terminal. This allows the charging of the capacitance by means of the power supply current. When the latter is higher than the current which is necessary for charging the capacitance in the normal operating conditions of the circuit, the current surplus is diverted towards the reference terminal via the second transistor whose base then receives a current which is complementary to the conduction current and has a value which is sufficiently large to start the conductance of this second transistor. If the potential of the input terminal becomes lower than that of the output terminal, the base of the second transistor receives a current which is sufficiently large to enable this transistor to convey the total quantity of power supply current, while the conduction of the first transistor, and thus the charge of the capacitance, are then interrupted.
This operation theoretically enables the power supply circuit to supply an output voltage which can be used for power supply purposes, while pulling a power supply current of a constant value. Indeed, if the value of the current pulled were varied, this would cause a change of information which, in a telephone apparatus, are materialized by the AC component of a current, referred to as line current, from which the power supply current is taken.
However, it is known that the gain of a transistor is dependent on the value of the current flowing through its collector-emitter path. A strong variation of the current flowing through the first or second transistor thus involves a considerable variation of the instantaneous gain. If the bias current is maintained constant, this means that the current used by the power supply circuit can be caused to vary. Moreover, a variation of collector-emitter voltages of the first and second transistors due to variations of the potential of the input terminal introduces, due to the Early effect, variations of the values of the currents flowing through these transistors. These phenomena cause variations of the value of the current used by the power supply circuit and thus disturb the information conveyed by the line current, which is not acceptable.